1. Field of the Invention
To reduce emissions and increase the efficiency of internal combustion engines with direct fuel injection, one goal is to inject the fuel into the combustion chamber of the engine in as finely-atomized a form as possible. To that end, first the injection pressure at which the fuel is injected through the fuel injection valve is increased. Second, the number of injection ports of the fuel injection valve is increased, so that the diameter of the individual injection ports can be reduced. The goal of this provision is to increase the energy of injection streams while at the same time reducing the droplet diameter. If very small quantities are to be fed, then when the pressures at the fuel injection valve are high the injection times are quite short. The course of combustion is consequently powerful and correspondingly noisy.
2. Description of the Prior Art
From European Patent Application EP 0 470 348 A1, for instance, a fuel injection valve of variable injection cross section is known, in which two rows of injection openings are embodied. These injection openings are controlled by an inner valve needle and a tube surrounding the valve needle; both the tube and the inner needle are acted on by closing springs, which press them into contact with a valve seat, as a result of which the injection openings are closed. If fuel at high pressure is introduced into corresponding pressure chambers, then the tube and the inner needle are acted upon by the fuel pressure in these pressure chambers. Depending on the pressure of the fuel introduced, either only the inner needle lifts from the valve seat and uncovers the first row of injection openings, or the inner needle and tube lift up from the valve seat successively, so that both rows of injection openings are opened in succession. The opening of the inner needle and the tube is accordingly pressure-controlled, so that the successive opening of the inner needle and the outer tube is achieved by means of a skillful design of the pressure faces and of the force of the closing springs.
Stroke-controlled fuel injection systems are also known from the prior art, in which a valve needle has a pressure face that is constantly urged in the opening direction by fuel at high pressure. The contrary force is generated not by a closing spring but rather hydraulically by a valve piston, which acts on the valve needle and in turn, because of the fuel pressure in a control chamber, exerts a closing force on the valve needle. As an example here, German Patent Disclosure DE 198 27 267 A1 can be named. By varying the fuel pressure in the control chamber, the closing force on the valve needle is changed, so that this needle is moved against the pressure face by the hydraulic force. Such stroke-controlled fuel injection systems are used in many modern internal combustion engines, especially for self-igniting engines in passenger cars.
A combination of the two systems, that is, of the variable injection cross section and the stroke-controlled injection system, would be especially advantageous to further optimize the combustion process. Until now, however, it was not possible without major effort to adopt the variable injection cross section to the stroke-controlled systems without making further modifications. Doing so requires complicated sealing edges or additional control valves, which are complicated to manufacture and expensive.